Helper-virus independent, E1 deleted adenovirus (Ad) based gene transfer vectors exhibit many positive attributes, including a large transgene encoding capacity, a relative ease of high titer production to clinical grades, and the ability to infect a wide range of tissue types. Despite the fact that [E1−]Ad vectors are significantly blocked in their ability to replicate (relative to a wild-type Ad), low level replication and/or gene expression derived from [E1−]Ad vectors can limit their usefulness (Amalfitano et al., (1998) J. Virology 72:926; Steinwaerder et al., (2000) Hum. Gene Ther. 11:1933). To overcome this problem, it has previously been demonstrated that [E1−]Ad vectors incorporating additional deletions in the Ad E2b genes (polymerase and/or pTP) rendered [E1−,E2b−]Ad vectors truly replication incompetent (Amalfitano et al., (1998) J. Virology 72:926; Hodges et al., (2000) J. Gene Medicine 2:250; Hu et al., (1999) Hum. Gene Ther. 10:355). As a result, [E1−,E2b−]Ad vector derived late gene expression was also significantly diminished, since Ad late gene expression is only initiated after Ad genome replication has occurred (Thomas et al., (1980) Cell 22:523).
Despite the problems associated with Ad replication, an Ad vector that can replicate its genome to high levels in infected cells would be valuable in certain applications. For example, this feature may be capitalized to amplify expression of a transgene encoded by the vector, and/or to induce cytopathic effects as a consequence of high level Ad replication and/or infectious virus production. For example, [E1a+,E1b−] Ad vectors have been described; the E1b deletion restricts E1a dependent vector replication (and generation of infectious vector) to cancer cells, resulting in their death (Bischoff et al., (1996) Science 274:373; Heise et al., (1997) Nature Med. 3:639). There is evidence, however, that [E1a+,E1b−]Ad vectors can also replicate in non-cancerous cells, potentially limiting the benefit/risk ratio of [E1a+,E1b−]Ad based cancer therapies (Rothmann et al., J. Virology 72:9470).
In a recent attempt to address the latter concerns, Ad vectors have been developed that are protease deleted (Oualikene et al., (2000) J. Virology 11:1341). Protease deleted [E1+]Ad viruses can replicate, but are blocked in their ability to produce infectious virus, due to inadequate maturation of viral capsid proteins during the late phase of the Ad life cycle. Importantly, however, [E1+]Ad vectors having deletions in both the E1b and protease regions are fully capable of producing wild-type levels of the Ad late genes once replication has occurred (Oualikene et al., (2000) J. Virology 11:1341). The late genes are numerous and include the hexon, 100K, penton, and fiber proteins. The toxicity normally associated with the expression of these proteins (particularly penton) may limit the overall usefulness of both [E1a+,E1b−]Ad and protease deleted [E1+]Ad vectors.
Accordingly, there is a need in the art for improved Ad vectors, in particular, improved replicating Ad vectors.